The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion components that include an amplitude component and a phase component are added to the original signal. These additional components are due to the amplitude compression characteristics (AM/AM) and the phase distortion (AM/PM) characteristics of the RF power amplifier when it is driven over a range of amplitudes. If these distortion components are not compensated they will cause spreading of the spectrum into the adjacent channels and thus interfere with communication devices using adjacent channels.
A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a feed forward correction circuit in a feed forward amplifier, also known as post-distortion. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier to create an error signal. The error signal is then amplified and added to the RF power amplifier's output with an amplitude, phase, and delay adjusted for maximum cancellation of the distortion components. However, the amount of distortion reduction available in a feed forward amplifier is limited by the distortion introduced into the amplified error signal by an error amplifier.
In order to compensate for the distortion introduced to the error signal, a technique of pre-distorting the error signal was proposed by Cova, et al., in U.S. Pat. No. 6,104,241. Cova includes a traditional feed forward amplifier in which an RF input signal is amplified by a main amplifier to produce an RF output signal that includes distortion components. The RF output signal is sampled to produce a sampled output signal that is fed to a feed forward correction circuit. In the feed forward correction circuit, the sampled output signal is combined with a delayed sample of an RF input signal in order to produce an error signal that comprises isolated distortion components. The error signal is then fed to an error amplifier that amplifies the error signal and feeds the amplified error signal into a combiner at the output of the main amplifier. The combiner combines the amplified error signal with the main amplifier output signal, thereby providing cancellation of distortion components in the main amplifier output signal.
In order to compensate for distortion introduced to the amplified error signal by the error amplifier, Cova provides a predistorter at an input to the error amplifier. Samples of the main amplifier output signal and the RF input signal are also fed to a distortion detector. Based on the main amplifier output signal and the RF input signal, the distortion detector detects a distortion energy of the main amplifier output signal and provides a measure of the energy to a digital signal processor (DSP). The DSP determines weighting coefficients based on the detected distortion energy of the main amplifier output signal, which weighting coefficients are used to control a variable attenuator and a phase shifter in the predistorter that predistort the error signal by adjusting a gain and phase of the signal, thereby compensating for distortion introduced to the error signal by the error amplifier and reducing the detected distortion energy of the main amplifier output signal.
A problem with the predistortion proposed by Cova is that it is a hardware implementation that operates at RF and predistorts an error signal by use of a variable attenuator and a phase shifter. Cova's predistortion loop is an adaptive feedback loop that continually feeds back the amplified error signal and continually modulates the error signal based on the fed back amplified error signal. This technique is expensive to implement, particularly as more weighting coefficients are generated, requiring additional hardware, in an effort to improve the accuracy of the amplitude and phase adjustment of the error signal in order to produce a predistorted error signal. Furthermore, the modulation of the error signal by a variable attenuator and a phase shifter introduces additional distortion and injects a variable group delay in the forward path of the error signal, which is undesirable in a feedforward error correction system. In addition, the technique proposed by Cova performs distortion correction of the error signal at RF. The informational content of the signal is the baseband signal that has been modulated onto an RF carrier. Precision may be lost when the baseband signal is first modulated onto an RF carrier and then the modulated RF carrier is corrected in main and feed forward signal paths with predistortion.
Therefore a need exists for a method and apparatus for minimizing the distortion introduced into an error signal by the feed forward correction circuit.